1. Field of the Invention
The present invention relates to syringes and more particularly concerns syringe assemblies with barrels having portions made of coextruded plastic materials.
2. Description of Related Information
Generally speaking, a hypodermic syringe consists of a cylindrical barrel, most commonly made of thermoplastic material or glass, having a distal end connected to a sharp needle cannula or adapted to be connected to a hypodermic needle assembly and a proximal open end adapted to receive a resilient stopper and plunger rod assembly. One of the purposes of the stopper is to provide a relatively air-tight seal between itself and the syringe barrel so that movement of the stopper up and down the barrel will cause liquid medication, blood or other fluids to be drawn into or forced out of the syringe through the distal end. The stopper is moved along the syringe barrel by applying axial force to the rigid plunger rod which is connected to the stopper and is sufficiently long as to be accessible outside of the barrel. The stopper should be sufficiently flexible so that it will seal the inside diameter of the barrel without requiring excessive force to move it up and down the barrel.
In order to assure an air-tight seal between the rigid syringe barrel and the resilient stopper, known prior art stoppers are manufactured with a larger outside diameter than the inside diameter of the syringe barrels they will be used in. The syringe-stopper combination is designed so that the stopper, when introduced into the syringe barrel, is compressed enough to provide adequate pressure between the syringe barrel and the stopper to seal this interface.
The resilient stopper, in prior art devices, should be chemically stable so that undesirable amounts of the various chemical components of the stopper do not enter the liquid contained in the syringe. Since hypodermic syringes are frequently used to inject medication into a human body or to withdraw blood for subsequent analysis it is not desirable to have stoppers introduce foreign substances which can affect the drug or the blood analysis. Hypodermic syringe stoppers are most commonly made of materials such as natural rubber or butyl rubber. Although the rubber stoppers have desirable physical properties they possess a number of disadvantages. For example, rubber stoppers contain additional chemical components such as fillers and vulcanizing accelerators which can exude to the surface and contact liquid in the syringe wherein blood test results or medication efficacy may be affected. Also, rubber stoppers are expensive to manufacture due to the long mold cycle time required by the vulcanizing step which takes place while the stoppers are in the mold.
Recognizing the above-mentioned deficiencies in rubber stoppers, it is desirable to provide a syringe assembly having a stopper or a piston made of plastic material. Normally, thermoplastic materials will be less expensive to manufacture due to shorter mold cycle times which result in improved productivity of the molding machinery. The possible effects of fillers and vulcanizing agents on the liquid contents of the syringe can be eliminated since these rubber additives are not necessary in the production of plastic stoppers or pistons. Also, the complexity of drug compatibility testing may be reduced when thermoplastic syringe stoppers are used in combination with a plastic barrel so that the stopper may be formed of materials having similar chemical properties to the barrel.
U.S. Pat. No. 4,500,310 to Christinger teaches an improved plunger rod design which allows the use of a resilient thermoplastic stopper. Christinger overcomes many of the cost and possible functional problems related to rubber stoppers but still requires an additional elastomeric stopper element to be connected to the plunger rod as in prior art syringes.
To fabricate a syringe using a rigid thermoplastic barrel and piston believed to be undesirable because the stresses produced by an interference fit substantial enough to provide an air-tight seal between the stopper and the barrel may result in a syringe assembly requiring an excessive amount of force to move the piston along the barrel to inject medication. Also, the tight fit between the barrel and the stopper, using thermoplastic materials, over a period of time may cause the piston and/or barrel to achieve a compression set. That is, the stresses of the interference fit between the stopper and the syringe can cause cold flow of the thermoplastic piston and/or barrel and thus the diameter of the stopper can become reduced, or the bore of the barrel enlarged so that the stopper may no longer effectively seal the contents of the syringe. Lubricants and expensive control of dimensional tolerances of the components can reduce some of these problems.
Although prior art syringes using rubber stoppers have served the medical community well over many years there are component cost and compatibility issues associated with rubber stoppers. Although the teachings of Christinger provide a substantial improvement by allowing the use of a thermoplastic resilient stopper there is still a need for a simple, straight-forward, reliable, easily fabricated syringe assembly having a plunger and piston made of low-cost rigid materials such as thermoplastics.